1. Field of the Invention
The present invention relates to cable testing devices and, more particularly, the present invention relates to a cable testing apparatus capable of testing a cable faster and utilizing fewer measurements than conventional cable testing devices.
2. Related Art
High speed data communications cables in current usage include pairs of wire twisted together forming a balanced transmission line. Such pairs of wire are referred to as twisted pairs.
One common type of conventional cable for high-speed data communications includes multiple twisted pairs. In each pair, the wires are twisted together in a helical fashion forming a balanced transmission line. When twisted pairs are placed in close proximity, such as in a cable, electrical energy may be transferred from one pair of the cable to another. Such energy transfer between pairs is undesirable and is referred to as crosstalk. Crosstalk causes interference to the information being transmitted through the twisted pair and can reduce the data transmission rate and can cause an increase in the bit error rate. The Telecommunications Industry Association (TIA) and Electronics Industry Association (EIA) have defined standards for crosstalk in a data communications cable including: TIA/EIA-568-A, published Oct. 24, 1995; TIA/EIA 568-A-1 published Sep. 25, 1997; and TIA/EIA 568-A-2, published Aug. 14, 1998. The International Electrotechnical Commission (IEC) has also defined standards for data communications cable crosstalk, including ISO/IEC 11801 that is the international equivalent to TIA/EIA 568-A. One high performance standard for data communications cable is ISO/IEC 11801, Category 5.
It is desirable that these high speed data communication cables, as well as the newly-emerging class of broadband cables based upon twisted pair technology are tested to ensure compliance with the various standards in a cost-effective and accurate manner. Testing a cable characterizes a cable using various measurements including: the attenuation of a particular twisted pair, the crosstalk between twisted pairs contained within the cable, and the impedance of each individual twisted pair. These measurements are generally performed using a network analyzer that may include a S-parameter test set. As used herein, a network analyzer includes cable testing devices utilizing a network analyzer individually or a network analyzer in combination with an S-parameter test set.
In conventional cable testing devices, each twisted pair or combination of twisted pairs is connected to the corresponding ports of the network analyzer test set. Conventionally, the network analyzers operate in an unbalanced operational mode and therefore require the use of balun transformers to drive the twisted pair cable in a balanced mode. Some conventional cable testing devices use a switch matrix to connect each twisted pair or twisted pair combination to the port of the network analyzer. The switch matrix may be placed on either the unbalanced side, i.e., between the network analyzer and the balun, or the balanced side of the balun, i.e., between the balun and the twisted pairs under test.
In order to quantify the level of crosstalk occurring between twisted pairs contained within the cable, the power-sum crosstalk for each pair, that is, the vectorially-added power induced by all the adjacent disturbing twisted pairs into the twisted pair being measured, must be determined. Thus, in conventional testers for each twisted pair, all of the possible pair combinations will have to be measured in order to determine the power sum crosstalk.
There are two forms of crosstalk measured in cable testing devices. The first is near-end crosstalk (NEXT). NEXT is measured in conventional devices by using a network analyzer as described above for each twisted pair within the cable. For a cable having 25 twisted pairs, the number of combinations of 25 twisted pairs taken 2 at a time means that 300 measurements must be made. Similarly, far end crosstalk (ELFEXT) is conventionally measured using a network analyzer also for each pair of cables. Similarly, this means that there will be an additional 300 combinations of the 25 twisted pairs taken 2 at a time to properly characterize the ELFEXT of the cable. Thus, in conventional cable testing devices, to properly characterize the twisted pairs within the cable for the NEXT and ELFEXT measurements, a total of 600 measurements are needed. Performing 600 measurements requires a substantial amount of time and concomitantly increases the cost of producing the cable.
What is needed, therefore, in the art is a cable testing apparatus that is able to perform the NEXT and ELFEXT measurements faster and easier than conventional methods.